1. Field of the Invention
The present invention relates to a polymeric foam material prepared from poly-N-arylenebenzimidazole. More particularly, the present invention relates to a cross-linked poly-N-arylenebenzimidazole material of especially low foam density.
2. Description of the Prior Art
Recently, several methods of preparing polybenzimidazole materials have been reported in which the linking units of the polymer have the following structure: ##STR1## U.S. Pat. No. 3,708,439 shows a method of preparing poly-N-arylenebenzimidazoles by reacting certain organic tetraamines with aromatic dicarboxylic acids or their phenyl esters. However, while foamed arylenebenzimidazoles can be prepared by this reaction, the reaction does not result in a polymeric material of especially low density. In the polymerization reaction the organic tetraamine and diphenyl ester starting materials are either melt condensed or reacted in the presence of an organic solvent such as phenol, m-cresol or the like. During the reaction, the reacting starting materials release phenol and water as volatile by-products which supplement the organic solvent as a blowing agent in the reaction. The release of these volatile materials necessitates the adjustment of the foaming schedule to compensate for the continuously slow production of the volatile by-products which complicates the reaction.
Another reference (U.S. Pat. No. 3,551,389) shows a technique of preparing polybenzimidazoles by melt condensing an organotetraamine initially with an aromatic dicarboxylic acid derivative such as the diphenyl ester or anhydride to form a prepolymer. Thereafter, the prepolymer is heated to an elevated temperature in the presence of added amounts of phenol, optimumly of about 10%, to complete foaming and polymerization of the benzimidazole prepolymer. However, the benzimidazole polymers are not of the N-arylene-type, and are not of especially low density.
Another reference (U.S. Pat. No. 3,784,517) shows a method of forming polybenzimidazole fibers in which an aromatic tetraamino compound is reacted with a dephenyl ester or anhydride of an aromatic dicarboxylic acid in a one-step process in a high boiling organic solvent such as sulfolane or phenyl sulfone. However, a foamed product of low density is not obtained and is only illustrative of a technique of forming polybenzimidazoles by a solution/slurry process.
Other techniques are known in which an aromatic dicarboxylic acid anhydride such as phthalic anhydride and an organic tetraamine such as 1,2,4,5 tetraaminobenzene are copolymerized to yield a foamed product (U.S. Pat. No. Re. 26,065). However, the polybenzimidazole product prepared by this procedure is not a low density foamed material, and is not of the N-arylene-type. A need, therefore, continues to exist for a foamed N-arylenepolybenzimidazole material of high melting and low density characteristics.